1. Field of the Invention
The invention relates to a method of fabricating multilevel interconnections of a semiconductor device, and more particularly to a method of fabricating a metal plug for multilevel interconnections.
2. Description of the Related Art
As the integration of semiconductors is enhanced, the dimensions of devices cannot supply enough area for interconnection. To match the requirements of the metal oxide semiconductor (MOS) devices with smaller dimensions, designs of multilevel interconnections are adapted in most of the integrated circuits (ICs). Normally, an inter-metal dielectric (IMD) layer is used to isolate two conductive layers. By the formation of a metal plug, the conductive layers are electrically connected. Moreover, to obtain a better dielectric characteristic, for example, the adhesion, a barrier layer made of a conductive material is formed between the metal plug and the conductive layer.
Titanium nitride is a very common material used as a barrier in a high density IC. To obtain a better ohmic contact between metal and silicon, a titanium nitride barrier layer is formed and assembled with a titanium layer as a titanium/titanium nitride layer. For example, for the process of fabricating a contact of an alloy, the whole metal structure is assemble by three different materials, titanium, titanium nitride, and alloys to decrease the work function, and the suppress the formation of spike and electromigration.
In a conventional process of forming a barrier and an interconnection structure, a titanium layer is formed. By physical vapour deposition, a titanium nitride layer having a thickness of about 800 .ANG. to 1200 .ANG. is formed on the titanium layer as a barrier layer. After a rapid thermal nitridation (RTN), a titanium nitride layer with a better contact with the titanium layer is formed. A refractory and conductive metal layer, such as a tunsteng layer, is formed. By performing an etching back process, a metal plug is formed. After cleaning the surface of the device, interconnection of another layer can be formed.
A conventional method is explained and shown on FIG. 1A to FIG. 1E.
Referring to FIG. 1A, on a substrate 100 having a MOS and metal layer formed thereon, a dielectric layer 102 is formed to isolate two metal layers. A better dielectric layer 102 is formed as a single-layered or double-layered spin-on-glass sandwich structured dielectric layer, which can prevent the voids formed during a pure chemical vapour deposition for forming a dielectric layer. Referring to FIG. 1B, using a mask to cover the dielectric layer 102, a photolithography process is performed to define a connecting channel for different metal layers. A contact window or a via hole is formed to expose the metal layer on the substrate 100. Referring to FIG. 1C, to enhance the adhesion between the tungsten plug formed subsequently and the other material, a titanium nitride layer or a tungsten nitride layer is formed before the formation of tungsten layer. Moreover, during the etching process, the titanium or tungsten nitride layer can be used as an etching stop layer due to the difference of plasma spectra between tungsten and titanium or tungsten nitride. Applying titanium nitride to the process metallization, titanium nitride is assembled with titanium as a form of titanium nitride/titanium layer. Before the formation of metal plug, a metal glue layer 106, such as a titanium layer, can be formed on the dielectric layer 102 and the circumference of contact window 104. By physical vapour deposition, a barrier layer 108 having a thickness of 800 .ANG. to 1200 .ANG. is formed on the metal glue layer 106. To improve the adhesion between the barrier layer 108 formed by titanium nitride and the metal layer 106, a rapid thermal nitridation is performed at a range of temperature between 550.degree. C. to 900.degree. C. Referring to FIG. 1D, a refractory and good conductive metal layer 110, for example, a tungsten layer, is formed on the barrier layer 108. The barrier layer 108 is covered by the metal layer 110, and the contact window 104 is filled by the metal layer 110. Referring to FIG. 1E, a part of the metal layer 110 is removed to form a metal plug 110a in the contact window 104. If the metal layer 110 is a tungsten layer, a gas source containing fluorine, such as a mixture of carbon fluoride and oxygen, a mixture of fluorine nitride and oxygen, or a mixture of fluorine sulfide and oxygen, is used as the etchant with the titanium nitride layer 106 as an etching stop layer. An etching back process is performed to form a tungsten plug 110a.
After the tungsten plug is formed, the subsequent processes, such as the deposition of another metal layer, photolithography and etching process, are performed to form the second level of interconnection.
In the above conventional process, the titanium nitride grain of the barrier layer is not uniform. Thus, during the deposition of tungsten, the reacting gas, tungsten fluoride (WF.sub.6) is reacted with titanium glue layer along the edge of the titanium nitride grain of the barrier layer to form titanium tri-fluoride (TiF.sub.3). An additional thermal process to remove the titanium tri-fluoride causes a volcano-like protrusion on the barrier layer. On the volcano-like protrusion, tungsten residue is left in the subsequent etching back process of tungsten.
In addition, the non-uniform titanium nitride grain further causes the deposited tungsten grain to be non-uniform. The larger size tungsten grain cannot be removed by the subsequent etching back process. The residue of tungsten is thus left.